We have been attempting to analyze the latency of human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7) in vitro and in vivo.1) In vivo, study ; the latency and reactivation of HHV-6 and HHV-7 were surveyed in throat of children of different age-groups and adults by polymerase chain reaction. The detection rate of HHV-6 DNA was the highest in children aged 1-year-old and decreased with age, while the detection rate of HHV-7 DNA increased with age and reached a maximum in adults. HHV-6B was detected in almost all samples. When the antibody prevalence was determined in the 4 groups of children. HHV-6 antibody was detected in almost all children aged 6-12 months after birth, and children were infected with HHV-7 later than HHV-6.2) In vitro study ; in order to identify the regulatory genes of HHV-6 the whole DNA of HHV-6B was sequenced and one of immediate early genes was analyzed. The immediate-early protein IE 1 ; 958 amino acids of HHV-6B strain HST were expressed as b-galactosidase fusion proteins in E coli. Using Western blot analysis, an antigenic region of the IE 1 protein was mapped between residues 340 and 505. Monospecific antibody raised against the fusion protein reacted with apparent molecular weights of 155 and 170kDa, and was detected as granular fluorescence in nuclei of infected cells by IFA.3)By using the latency system in vitro, we have identified a latency specific transcript, which is found in monocyte/macrophage infected with HHV-6B.This mRNA was specifically transcribed from the region of immediate-early (IE) gene and found in the peripheral blood in normal children, suggesting that a mRNA is transcribed in monocyte/macrophage even in vivo.